Rheostat potentiometer



April 29, 1952 DEWAN RHEOSTAT BOTENTIOMETER 4 Sheets-Sheet 1 Filed March 31, 1950 54 1C1. l. Z0 Z1 Z3 Z3 55 34 38 40 IN VEN TOR. A 0/v Q5 WA/V April 29, 1952 DEWAN 2,595,189 RHEOSTAT POTENTIOMETER Filed March :51, 1950 4 Sheets-Sheet 2 T l a l U INVENTOR.

A o/v DE WAN April 1952 L. DEWAN 2,595,189

RHEOSTAT POTENTIOMETER Filed March 31, 1950 4 Sheets-Sheet 3 66 A ENE/ April 29, 1952 DEwAN 2,595,189

RHEOS'I'AT POTENTIOMETER Filed March 31, 1950 4 Sheets-Sheet 4 Patented Apr. 29, 1952 RHEOSTAT POTENTIOMETER Leon Dewan, New York, N. Y., assignor of onethird to Milton H. Feig, Brooklyn, N. Y., and one-third. to Frederick E. Hettling, Queens,

Application March 31, 1950, Serial No. 153,226

8 Claims.

This invention relates to an electrical resistance device and more particularly to a resistor which is slidably contacted to produce a rheostat or potentiometer. This shall generally be referred to herein as a rheostat.

A prime object ofthe present invention is to produce such a resistor of highly accurate linearity so that the resistance may vary uniformly in accordance with the movement of the contactor.

Another object is to produce such a resistor with a high accuracy of its total resistance.

Another object is to accomplish the above objects in an automatic manner suitable for the need of commercial production.

Other objects and advantages will [appear in the description.

A common form of such a resistor is a helix of resistance wire wound upon a core which itself is twisted into a helical coil to provide a long slide path. In this instrument linear accuracy is an important factor, especially when it is used as apotentiometer in conjunction with modern devices'such as servo-controls, etc.

Ordinarily to attain an accurate linearity the core is wound with resistance wire by special methods and the body of the instrument and the parts are precision machined. However, though the wire winding is made linearly accurate, while the core is straight, the twisting of the core into a helix, as well as the slight eccentricity in the body of the instrument and in the contactor assembly and other inaccuracies associated with production finally introduce a certain percentage of non-linearity when the instrument is finally assembled.

According to this invention, the components of the rheostat as well as the winding are made with no particular attempt at great accuracy. When an instrument has been assembled and is in working condition, it is subjected to a special "accuratizing process that removes all inaccuracies including those in the original winding and the components and those that crept in during the assembly.

The rheostat is so constructed that a longitudinal portion of its winding, facing the outside of the instrument, is exposed and accessible. This portion is then coated with an even layer of a conductive substance of much higher resistance than that of the winding, for example, carbon black or graphite dispersed in a plastic binding agent. This substance increases the conductivity of the winding to an extent oommensurate with the expected linear inaccuracy of the instrument. The rheostat is then mounted in an accuratizing device in which the contactor of the rheostat is advanced in conjunction with that of a master rheostat of high linear accuracy. Both of these instruments are connected in a servo-control network including a Wheatstone stored guides the linearity of the rheostat to conform to that of the master rheostat.

Fig. 1 is afront view of the improved rheostat, with part of the front wall cut away-to reveal the interior. 1

Fig. 2'is a cross sectional view taken on the plane of the line 2-2 of Fig. 1. e

Fig. 3 is an enlarged fragmentary perspective view of a portion of the winding core of Fig. 1, with part of the winding and plastic coating removed.

Fig. 4 is an enlarged perspective detail view showing a winding and an adjacent separator.

Fig. 5 is an enlarged detail view showing the contactor riding on the wire.

Fig. 6 is an enlarged detail view looking down on top of the accuratizing device.

Fig. 7 is a front view of the device of Fig. 6 with parts being shown in section and parts being cut away.

Fig. 8 is a detail view partly in section and partly in elevation showing the wall of the rheostat of Fig. 7 with its contactor and its relation to the cutter outside.

Fig. 9 is a sectional view of the parts shown in Fig. 8 looking upwardly from the bottom.

Fig.'10 is a plan view of the parts of Fig. 9 turned half way around to view the track on the surface of the rheostat.

Fig. 11 is a diagrammatic view of the electrical connections of the apparatus of Figs. 6 and 7.

Fig. 12 is a diagrammatic view showing the cutting conditions of the track in the first stage, on an enlarged scale.

Fig. 13 is a similar view showing the cutting conditions when the track is finally perfected in the second stage.

Fig. 14 is an enlarged detail sectional view showing a modified form of cutting instrument I and its use.

Fig. is a view of Fig. 1% turned half way around.

Fig. 16 is a plan view showing the mechanism on one side of the master rheostat panel.

Fig. 17 is a bottom plan view of the mechanism of Fig. 16. i

Fig. 18 is a bottom plan view of the mechanism of Fig. 17 showing the resistors and switches on the reverse side of the master rheostat panel.

Referring in particular to Figs. 1 to 5, a resistance wire 20 is wound on a core consisting of a central portion 2| of copper or other soft metal, and a plastic coating 22. The winding core is wound on a polished metal-mandrel side'by side with a separator strip consisting ofa metal core 23 having a concave groove on the underside, with a plastic cover 24. The winding core and the separator are cemented together as are the plastic end rings 25. The space on the winding 20 is then filled with a conductive paste, preferably containing carbon black or graphite dispersed in a plastic with solvent for forming the "track 26. It is preferred that all the plastic-used including the cement have the same solvent to form a unitary structure on drying. The mandrel is then withdrawnand the formed cylinder mounted on the annular projections .21 of the metal end pieces 28 and 29,which also carry the bearings 30 for the shaft 3|. This shaft turns'the carrier 32 by means-of thekey 33 flxedin the shaft andslid- 'ing in a keyway in the carrier. Theroller34 mounted on a support 35 projecting from the carrler-and engaging the groove/23a: in the core causes the carrier to slide along the shaft-and follow the helical winding 'when the shaft is turned. On'the support 35 projectingfrcm'the carrier is hinged'thearm 31 carrying a metallic roller 38 having a ball bearing and gear like grooves in its periphery.

The spring 39 on the hinge urgesthe roller'33 against the winding'to act as'a contactor. The

gear-like grooves on'the roller 38 are-of such size and spacing as to engage the wires of the winding in gear fashion, more easily seenin Fig. The result is sharp definition and a certain amount of wipe for electrical contact with very little of the friction or abrasion that occurs in the case of-a solid slide contactor. The torque for turning the shaft is small even when the'pressure of the contactor against the winding is ample for positive electrical contact at all'itimes.

When the rheostat has been assembled thus far, it is mounted in an aceuratizing device (later described) and processed, after which the eylindrical plastic shell 40 and the flanged metal barrel 4| are put into place and the barrel flanges bolted to the end pieces 28 and 29 as "shown.

Figs. 6 to 10 illustrate the accuratizing device. The rheostat minus the plastic shell 40 and the barrel 4| is bolted by means of the end piece 28 to the table top of the upright pillar 42 whose lower portion 43 contains projecting screw threads of the same pitch as the helix of the rheostat. The pillar is fixed in the base 44. A threaded hub 45 with a flanged top 46 engages the screw 43 and slides on the pillar 42. The hub 45 is turned by the worm gear 41 whose hub 48 en gages hub 45 through the key 49 which slides in a key way in hub 48. Thehub 48 .restson "the 'rim of the cylinder 50 and the worm gear is rotated by the mating screw 5| mounted on the shaft 52 which is .rotatedby the motor 53. The motor drives a highly accurate master rheostat 54 (described later) through a change gear box a small abrasive cutter 64 at its end. The cutter is operated in the wall of the cylinder at a depth very-close tothe surface of the winding 20. theassembly on flange 45 turns in a helical path due to the screw, the cutter 64 forms a groove that parallels the-graphite track 26. As the cutter is actuated vertically by the servo mechanism 59, the groove wavers and cuts to varying extent into the track 26 to vary its width and thereby effect the conductivity of the winding beneath the resultantly modified portion of the track.

This is shown: more in .detailin Figs. 8, 9 and 10 which show'top,'side and front-view, respectively, of the portion of the winding and the graphite track being modified. It willbe seen in Fig. 8 that the contactor 3B touches the winding 20, at the samepoint, approximately, as the-edge of the cutter 45 as indicated by the line 33. Thus the track is continually modified at a-pcint where the contactor 38 makes electrical connection between it and the active "terminal 65 oi-the rheostat, the other end of the winding beingfree.

The pillar 42 -may be hollow (notshown) .to allow wires from the rheostat to lead .to outside connections. The electrical connections from the motor and the servo mechanism are led to slip rings temounted on the insulating ring 81. Theseislip rings are contacted by brushes on the brush assembly 68. This assembly rides on the vertical square rod .693 fixed in the base. .The rollers 10 attached .to the brush assembly engage the ring H and cause the brushes to rise and fall with the flange-wand keep thebrushes contacting the .slip rings. Theflexible electric cable lZ'ca-rries the connections to the electric system;

Referring now to Fig. 11 showing the electrical system, terminal E5. of therheostat and the contactor are connected in a Wheatstone bridge circuit with the equal balancingresistors 13. Voltage is applied to .theibridge at the terminals 14. The balance terminals 15 are connected to the input of the special amplifier 1.6 whose output may actuate the motor of the servo mechanism 59 through the switch H. The amplifier'lli converts any unbalance of the bridge into currents whose polarity depends on thedirection of unbalance so that the servomechanism moves the cutter up or down accordingly. This type of amplifier is well known and needs no detaileddescription. The conditions of the accuratizing process are as follows:

The rheostat is wound toa resistance higher than the resistance finally desired, by a percentage somewhat greater than the expected percentage of linear inaccuracy of thewinding. "The graphite substance forming thelayer is of such resistance (considerablyhigher than the wind ing) that it lowers the resistance of the. winding to a point not quiteas far below the-desired resistance valueas the winding resistance is above the same.

As an example, for the sake of explanation, if the desired final resistance is .100 ohms per inch of winding. and the linear inaccuracy of the windresistance of 101 to 102 ohms per inchand the graphite layer which may be approximately 4500 ohms per inch resistance reduces the winding to a mean resistance of approximately 99 ohms or more per inch.

In the first accuratizing process the motor 53 is engaged to drive the mounted rheostat and the master rheostat in unison. The master rheostat would, according to the example given 7 above, increase 100 ohms per every inch that the contactor 38 traverses the rheostat winding. Then when the rheostat is conforming to the ideal linearity of the master rheostat, the cutter 64 would be constantly removing enough of the graphite track to reduce its conductive effeet to a half. In practice it is desirable that the track be of greater resistance than the theoretical 4500 ohms given above so that the extent to which the cutter 64 digs into the track under ideal conditions is somewhat less than half of the area of the track as shown by the line 18 in the region '19 of Fig. 12. When the linearity of the rheostat winding 20 departs from the ideal linearity of the master rheostat the bridge is unbalanced and the cutter consequently is moved by the servo mechanism to vary the extent of the cut into the track. As this happens, it

, Thus as the winding 20 is traversed from one end to the other it is constantly corrected toconform to the linearity of the master rheostat to keep the bridge balanced.

In practice, it is preferable that an ample allowance be made for the linear inaccuracy so that the resistance of the wire winding itself would be made 103 ohms or more per inch and the conductivity of the graphite track be made correspondingly greater in relation thereto. Thus the linear inaccuracy would not require that the whole width of the track be modulated for correction but a margin would be left, as indicated-in Fig. 12 by the dotted line 80 which represents the limit of the depth required for correction'in this case.

In cases where it is an advantage to reduce" or eliminate the temperature versus resistance change in the instrument, the proportion of conductivity of the track to the windng may be in creased even more than indicated when necessary. Graphite has a high negative temperature coefiicient of resistivity, about -.0026. If the winding is made of a resistance wire having a positive coefiicient of about +.00014 (this is found for example in the alloy 80 Ni 20 Ch), and if the track is made of such relative. conductivity that after accuratizing it represents roughly between 5% and 6% of the total, the proportional value will be such that with a given rise of temperature the track having the greater negative coeificient would cause a decrease of resistance caused by the lesser positive coefficient of the winding. The instrument would thus have a practically constant resistance under any temperature changes in ordinary use since the temperature versus resistance scale of these two conductors is linear for about 200 C. above and below zero.

The first accuratizing process described leaves a slight linear inaccuracy because when the bridge is unbalanced and actuates the cutter to change the winding resistance which' intum tends to restore balance, the action is not instantaneous, there is a certain amount of lag and also a certain amount of under correction since any attempt at perfection could cause the action to overshoot and result in oscillation or instability. To bring the rheostat-nearer to perfection the winding is retraversed under the following conditions.

The switch 8| is closed. Switch 11 is put into midway position to disconnect the servo mechanism. The switch 82 is pulled upward and the clutch 51 is caused to engage the motor 53 to the recorder 56. The output of the balance amplifier 16 now feeds to the input of the amplifier 83 while the output of same connects to the recording input of recorder 55. The motor 53 is reversed. During this retraversal the linear inaccuracy remaining from the first process causes unbalance of the bridge and this unbalance is recorded on the magnetic tape as a modulation, the median line of which corresponds to the zero balance of the bridge. When the traversal is completed, the switch BI is opened, switch 11 is pulled to the right to connect the output of amplifier 83 to the servo mechanism, and the switch 82 is pulled down to connect the reproducing output of recorder 56 to the input of am The motor 53 is again reversed and the winding retraversed. During this retraversal, the record of the linear inaccuracy is applied to correct the same inaccuracy that produced the record. The output of the amplifier 83 is a modulated D. 0. whose median line or strength is of such magnitude as to cause the'cutter 64' to press against the tracks original median'line 18' in Fig. 13 with a pressure suificient to make a new median line 84. As the modulation departs from median the pressure on the cutter is varied and the depth of the cut is varied accordingly to correct the linear inaccuracy that produced the record.

It may be seen that the linear inaccuracy of a rheostat can be reduced by subjecting it to r the second or recording accuratizing process alone whereby the gross inaccuracy is recorded and played back to correct itself. However it is difiicult to gauge the magnitude of the play back to full accuracy and the change of conductivity of the track with depth of cut is slightly non-linear. It is the combination of the two processes that produces the higher linear accuracy since any shortcoming in the second recording process amounts to only a small percentage of the already small percentage of inaccuracy remaining from the first process. In cases where the accuracy of the total resistance value of the rheostat is of primary importance, the first process alone may be used since the median line in the graphite track then corresponds to the absolute resistance of the master rheostat and the rheostat would then have a very accurate resistance tolerance.

The change gear box 55 allows the master rheostat to function for rheostats of different re- I sistances by varying the rate at which the master rheostat resistance increases per given advance of the contactor on the rheostat. A similar result can be produced by varying the resistance of one of the balance resistors '13 in relation to the other to establish a desired proportion of the bridge. The gearing is desirable for establishing the larger steps in resistance and the latter for the smaller steps and graduated changes produced by varying the resistemce co! :one :of :tbe balance resistors '13 in .reilation "to {the :other ':to {establish :a .desired pro- ;portion of the bridge. .The gearing issuitable iioriobtaining .the;-.larger steps in the resistance :scale while .varying the proportion "of resistances 1:3 :can ."be used teachiere the smaller steps and :a Jfine graduation.

lsand 15 .illustrate an alternate type of cutter especially useful where the linear inac- -.curacy of a'winding variesshanaly' within short distances. The axis-of the thin abrasive wheel JSSl'oughl-y parallels the length of the track 28' in. ithisacase. Theshaft-BS' is moved byithe servo mechanism to cause the wheel-t modulate .the 'ztrack assshown by the -.two. ;arrows in .Fig. :14 and thus-varyitsarea. The traveliof the wheel rel- :ative :to the track is indicatedby'thearrow in :15. .It will :benoted that the fine edge of the wheel .may sharply define its .positi0n .and v-'make it possible :to correct inaccuracies occurring :in a'short space.

;.In Figs. 16, 176,116,218 which are reantop and front views, respectively, the shaft88 drives the shafts-81 and 88, respectively, through elements of aGeneva movement so that each revolution of shaft 86 causes shaft 81 to index by one sixth .of a'turn, andeach revolution of shaft 81 causes shaft 88 to index similarly, and soon, the r0- tation of shaft 86 being continuous. Thedisk- .liltemember 89 on =shaft 86 carries thepin ,98 which actuates the slotted wheel 9| by one sector for every revolution of the shaft 85. This ,typeof movement iswell known. The member 82con shaft 8'lcarries the pin Biwhich actuates the slotted wheel Slsimilarly.

".TheLShafts turn in bearings in 'theinsulating panel 95 and .on-the other side of thepanel they 'turn 'the collars 196 which are insulated from the shafts by rings .8"! of plastic material. The .rings :91 are-bonded to the shafts'and screws 98 in the :collars 95 maintain the collars securely in position. The contact rollers 99, I50 and 18! ,are.mounted on shafts 582 which are brazed to the thin flexible conductivestrips i l-3 attached tothecoH'arsSS. The contact arms Hi4 coming from the collars 95 contact the conductive arms Fl 05 which act asterminals for the'contactrollers.

Onpanel 95 is mounted ail-insulating annular projection I06 carrying ina circular groove the :resistance wire Hl'l WhiChlfOll'llS an almost complete circle, one ,end of whichis connected to the terminal I08. The wire .lfll is'in constant contact with the roller 99 which is pressed against it by the spring action. of the strip H23. Thelconductive arcuate segments 18$ overwhich theroller 99 rides form parts of a circular switch for the equal segments of the resistance H0. Segments i H are similarly connected to resistancell2. The'resistanceof the wire I8? is equal to one of the segments of resistance .l I 0 :and the resistance of H0 plus the ..resistance-of wire 1.81 is equal to one of the segments .of resistance H2. The roller llll :is connected to the terminal H3. One end :of resistance wire 181 is connected to roller me through terminal 108 and one end *of resistance H8 is connected to .resistance H2 and the roller '99 connected to terminal 114.

It "will thus be seen that .as the shaft 36 .is drivenzand the ,rollersfi travels around that the total resistance between the terminals H3 and 114 will be varied smoothly according to the resistance of the wire I87. As the roller '99 "bridges the gap between the ends of the wire (01 (see arrows indicating the rotation of the rollers) the Geneva movement acts and criedditional segment of resistance .I I0 is switched into circuit. As the roller 1.80 straddles those of segments 189 which connect to the ends .of

resistance I ll) anadditional segmentof. resistance H2 is switched into circuit thus vmaintaining the progressive change of the total resistance. In the drawings, the rheostathas reached the resistance of resistance H0 plus that of wire switch one segment of resistance H2 .into the circuit. Tosecure this simultaneous effect, the gaps i lilwhich separate the ends of all resistors are in line .to be bridged simultaneously .bythe rollers while the remaining gaps in any switch are slightly displaced.circularlyinone direction from the exact index line in reference to the gap lilti so that they are reached by the roller at a diiferent time than the gaps H5. Thus whenever main resistor is shorted by a roller bridging a gap H5 a segment of a main resistor of the next highest order is simultaneously introduced into the circuit to maintain an uninterrupted progression.

Although the device'as shown is based on a 60 turn, it may be based on a decade interval of 36 if desired. Additional resistances of a progressively higher order may be coupled to those shown in a similar manner.

I claim:

1. A resistor comprising an elongated resistance wire winding, a conductive substance of considerably higher specific .resistance than the wire winding covering said winding, the conductive substance varying in conductivity .linearly in accordance with but oppositely to the linear resistance inaccuracies of the wire .re-

sistor.

2. The method of producing an accurate rheostat consisting in forming an elongated resistor, assembling said resistor into the form of arheostat including the contactor means therefor and subsequently modifying the conductivity of said resistor variously throughout its length in accordance with but oppositely to the linear resistance inaccuracies of the resistor.

3. The method of producing an accurate rheostat consisting in forming an elongated resistor, assembling the same into the form of a rheostat including contactor means therefor, operating said rheostat by advancing the contactor means along the resistor and producing local variations in the conductivity of the resistorin proximity of the moving point of contact between the contactor and the resistor, said variations being made in accordance with but oppositely to the linear resistance inaccuracies of the resistor.

4. The method of producing an accurate rheostat consisting in forming an elongated resistor, assembling the resistor into the form of a rheostat including the contactor means therefor, operating said rheostat by advancing the contactor along the resistor, simultaneously advancing a cutting means along the resistor in proximity to the point of contact, said cutting means being applied to the resistor whereby the conductivity of the resistor is reduced, the application of the cutting means being modified in accordance with the linear inaccuracies of the resistor but in opposite manner thereto whereby said linear inaccuracies are substantially reduced.

5. Themethod of increasing the .accuracyof a rheostat consisting in recording the linear resistance inaccuracy of the rheostat subsequently, reproducing said record, and modifying the conductivity of the rheostat linearly in accordance with said reproduction whereby the linear resistance inaccuracy of said rheostat is substantially reduced.

6. Apparatus for increasing the accuracy of a rheostat comprising means for producing a record of the linear resistance inaccuracy of a rheostat, and means including record reproducing mechanism for transferring said record to the rheostat in such manner as to modify the conductivity of the rheostat linearly in accordance with the record whereby said linear resistance inaccuracy is substantially reduced.

7. Apparatus for producing an accurate rheostat comprising a support for a rheostat, means for advancing a contactor over the rheostat resistance element, a circuit for said resistance element, means for deriving an electric current from said circuit modulated according to the resistance inaccuracies of the resistance element, means actuating the resistance element for correcting said resistance inaccuracies and means whereby the electric current controls the correcting means directly in accordance with the modulations induced by the resistance inaccuracies to correct the same. i

8. The method of producing an accurate rheostat consisting in forming an elongated resistance element, moving a contactor over said resistance element, deriving an electric current modulated according to the linear resistance inaccuracies of the resistance element, and controlling by means of the electric current a correcting means applied to the resistance element whereby the resistance inaccuracies are corrected in accordance with the modulation they impose upon said electric current.

LEON DEWAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Re. 10,944 Weston July 17, 1888 1,859,930 Miller May 24, 1932 1,962,438 Flanzer et a1 June 12, 1934 2,366,614 Hansell Jan. 2, 1945 2,412,619 Kindermann et a Dec. 1'7, 1946 2,500,605 e Lange et a1 Mar. 14, 1950 

